Roller reamers have been used in downhole drilling operations for many decades to improve borehole quality. During drilling operations, the drill bit can be subject to wear causing the dimension of the drilled borehole to vary with time. Vibration of the bottom hole assembly (BHA) can also result in a borehole having many imperfections. Moreover, imperfections (such as ledges) and diameter changes can be introduced as the bore hole traverses a boundary between strata having differing mechanical properties. To improve borehole quality and consistency (e.g., to obtain a borehole having a consistent diameter), one or more roller reamers are commonly deployed in the BHA above the bit.
A conventional roller reamer includes a number of rotational cutting assemblies (e.g., three) deployed about the circumference of a tool body. Each cutting assembly includes a cutting or crushing roller deployed about a shaft (or pin) which is in turn coupled to the tool body. The rollers are configured to rotate about the shaft such that they rotate on the shaft and “roll” about the borehole wall during drilling. Such “rolling” reduces frictional forces between the BHA and the borehole wall which in turn reduces, torque, stick slip, and other vibrational modes. The rollers also include a number of cutting/crushing elements deployed on an outer surface thereof such that they cut (or crush) the local formation. Such cutting is intended to smooth the borehole wall and produce a borehole having a consistent diameter.
As is well known in the art, downhole tools are subject to extreme conditions, including mechanical shock and vibration (particularly radial compressive shock), high temperature and pressure, and exposure to corrosive fluids. These extreme conditions can result in numerous tool failure modes and generally require a robust tool design. For example, a robust sealing mechanism is required to prevent ingress of contaminants into the interior of the roller assembly and to prevent loss of lubricants. Seal failure can cause the roller to seize thereby significantly increasing the frictional forces between the BHA and the borehole wall. Such failures commonly require that the failed tool to be tripped out of the well. Moreover, in underguage holes, excessive radial forces on the roller assembly can cause numerous mechanical failures, for example, including fatigue cracking of the shaft and other internal assembly components. As a result of the aforementioned extreme conditions, it is sometimes desirable to service a roller reamer between drilling operations (or during a routine trip out of the wellbore). Such service may include, for example, replacement of the rotational cutting assemblies. A tool configuration that promotes such serviceability can be advantageous.